Pressurized gas injection between a molten or cooling plastic and a molding tool wall has been used as a technique for improving the surface quality of molded articles. For example, German patent document DE 195 16 290 C2 describes a plastic molding process in which, after the introduction of the plastic melt into the mold cavity, pressurized gas is injected through a fluid injection nozzle, so that a fluid blanket is created between the cavity wall and the plastic. A gas nozzle with a gap-shaped opening is used for this injection. Constrictions or wings are provided to limit the spread of the gas blanket.
Another process is known from WO 90/06220. A quantity of plastic melt, sufficient for the formation of a molded part, is introduced into the cavity of an injection molding tool. It is recognized that the plastic is subject to shrinking during the cooling process, and consequently, sink marks will typically occur. In order to avoid this, the pressurized gas is also introduced here, between the cavity surface and the molten plastic, as a blanket. Thus, the blanket is built up in a targeted manner and can be later removed.
WO 90/06220 describes two solutions for controlling the gas blanket. First, valves are provided that make possible gas sealing. By actuating, i.e., axial adjustment, of the valve shaft, the valve is selectively converted between the "open" and "closed" positions. Thus, the desired gas blanket can be created and removed in combination with the appropriate gas control. Alternatively, a porous sintered metal can be used. At the transition site in the molding tool where the gas blanket will be produced, a porous metal element is provided, through which pressurized gas, but not molten plastic, can flow. Therefore, targeted gassing is achieved without backflow of molten plastic. Detailed information on control of the extent of this gas pocket, however, is not found in the publication.
Others, however, have proposed techniques to control the size of the gas pocket. WO 93/01039, like DE 195 16 290, concerns the creation of the gas blanket between the cavity surface and the molten plastic and its restriction such that the blanket does not spread undesirably. The solution is to roughen the cavity surface where the gas blanket will be constructed to facilitate gas diffusion, and form smooth cavity surfaces elsewhere.